The present disclosure relates generally to switched mode power supplies with primary side control.
Power supplies, as needed for most electronic apparatuses, convert the electric power from power sources, such as batteries or power grids, into the electric power with specifications required by loadings. Among conventional power supplies, switched mode power supply, known to be compact in size and efficiency in power conversion, is globally popular, especially in consumer market.
Two different control methodologies are employed for switched mode power supplies. One is primary side control (PSC), and the other secondary side control (SSC). SSC utilizes a detection circuit directly sensing an output node powered by the secondary winding of a power supply, and the detection result is passed, via a photo coupler, to a power controller located in the primary side to regulate the power a primary winding converts. Different to SSC, PSC directly senses a reflective voltage across an auxiliary winding to indirectly know the output voltage over the secondary winding and the output voltage on the output node. For PSC, detection of output voltage and control of power conversion are both performed in the primary side. In comparison with SSC, PSC is cheaper in view of bill of material (BOM) cost, because it needs no bulky and costly photo coupler. Furthermore, PSC could naturally have higher conversion efficiency as it has no detection circuit located in the secondary side, which acts as an additional loading constantly consuming power.
FIG. 1 is a switched mode power supply 10 known in the art, employing the control methodology of PSC. Bridge rectifier 20 performs full-wave rectification, converting the alternative-current (AC) power source from a power grid into a direct-current (DC) input power source VIN. The voltage of the input power source VIN could have an M-shaped waveform or be substantially a constant. Via the driving node GATE, the power controller 26 periodically turns ON and OFF the power switch 34. When the power switch 34 is ON, the primary winding PRM of the transformer energizes. When it is OFF, the transformer de-energizes via the secondary winding SEC and the auxiliary winding AUX to build up output power source VOUT for loading 24 and operation power source VCC for power controller 26.
The voltage divider consisting of resisters 28 and 30 detects voltage drop VAUX over the auxiliary winding AUX, to provide the feedback node FB of the power controller 26 feedback voltage signal VFB. When the power switch 34 is OFF, the voltage drop VAUX is a reflective voltage in proportion to the voltage drop over the secondary winding SEC. Based on the feedback voltage signal VFB, power controller 26 builds compensation voltage VCOM upon the compensation capacitor 32, to control the duty cycle of the power switch 34 accordingly. Via current-sense node CS, power controller 26 detects current-sense voltage VCS, which represents the current IPRM flowing through not only the current sense resistor 36, but also the power switch 34 and the primary winding PRM.
FIG. 2 shows gate voltage VGATE, feedback voltage signal VFB, and secondary output current ISEC of FIG. 1, where the secondary output current ISEC is the current flowing through the secondary winding SEC and powering the loading 24. By knowing the peak value of secondary output current ISEC and the real discharge time TDIS-R when secondary winding SEC discharges, power controller 26 could conclude both the total amount of output charge from the secondary winding SEC and the average output current, to determine whether the average output current is out of specification.
As known in the art, an estimated discharge time TDIS-E, used as the real discharge time TDIS-R, is determined by sensing the first time when feedback voltage signal VFB drops across about 0V after gate signal VGATE turns to 0V. Nevertheless, estimated discharge time TDIS-E is very different to real discharge time TDIS-R, as shown in FIG. 2. After the completion of the discharge, it takes time for the feedback voltage signal VFB to reach 0V, causing the difference between the real discharge time TDIS-R and the estimated discharge time TDIS-E. This difference could cause both misjudgment of the average output current from the secondary side and failure of average output current regulation for switched mode power supply 10.